Congestive heart failure (CHF) is a condition in which a damaged or overworked heart cannot pump adequately to meet the metabolic demands of the body and/or can do so only with an elevated ventricular diastolic pressure. CHF is a major health problem worldwide, affecting millions of patients and accounts for numerous hospitalizations. Overall, the cost of treating CHF is very high (billions of dollars annually) and involves numerous physician visits. From 1979 to 1999, CHF deaths increased 145% and hospital discharges increased 155%. Survival is poor with 20% dying within one year and only 50% of patients surviving more than 5 years. The many patients suffering from this progressive, fatal disease tend to have an extremely poor quality of life and become increasingly unable to perform routine daily tasks.
Left ventricular (LV) filling pressure is a key factor in the progression of CHF. LV filling pressure represents the diastolic pressure at which the left atrium (LA) and left ventricle (LV) equilibrate, at which time the LV fills with blood from the LA. As the heart ages, cardiac tissue becomes less compliant, causing the LV filling pressure to increase. This means that higher pressures are required from the LA in order to fill the LV. The heart must compensate for this in order to maintain adequate cardiac output (CO); however, increasing the LA pressure strains the heart and over time irreversible alteration will occur.
Left Ventricular End Diastolic Pressure (LVEDP) and Mean Left Atrium Pressure (MLAP) (see FIG. 1) are the primary factors physicians use to evaluate CHF patients. MLAP and LVEDP correspond directly with LV filling pressure and are easy for physicians to identify from LV pressure data. The physician's ultimate goal is to increase cardiac output (CO) while reducing LVDEP. Treatment methods include medications, lifestyle changes, pacemakers, and/or surgery.
The only current method for evaluating intracardiac pressures, including MLAP and LVEDP, is invasive, namely, a cardiac catheterization procedure. Catheterization, however, provides only a snapshot of pressure data, carries morbidity, and is expensive. Following diagnosis of congestive heart failure, physicians would prefer to noninvasively monitor heart condition on a continuing basis in order to optimize treatment. Currently, no technology exists that has this capability.
Furthermore, in certain cases, CHF is complicated by mitral stenosis. This is a very complex situation and requires significantly more precise and continuous pressure data. Atrial fibrillation can develop as a result of this condition, and catheterization to evaluate such cases is considerably more complex since pressure gradients across the mitral valve must also be measured. Doppler echocardiography can be used to evaluate this condition; however, echocardiography again requires a specialized laboratory with specialized equipment and doesn't provide continuous measurements.
The treatment of cardiovascular diseases such as Chronic Heart Failure (CHF) can be greatly improved through continuous and/or intermittent monitoring of various pressures and/or flows in the heart and associated vasculature. Porat (U.S. Pat. No. 6,277,078), Eigler (U.S. Pat. No. 6,328,699), and Carney (U.S. Pat. No. 5,368,040) each teaches different modes of monitoring heart performance using wireless implantable sensors. In every case, however, what is described is a general scheme of monitoring the heart. The existence of a method to construct a sensor with sufficient size, long-term fidelity, stability, telemetry range, and biocompatibility is noticeably absent in each case, being instead simply assumed. Eigler, et al., come closest to describing a specific device structure although they disregard the baseline and sensitivity drift issues that must be addressed in a long-term implant. Applications for wireless sensors located in a stent (e.g., U.S. Pat. No. 6,053,873 by Govari) have also been taught, although little acknowledgement is made of the difficulty in fabricating a pressure sensor with telemetry means sufficiently small to incorporate into a stent.